Glycitein is an O-methylated isoflavone which accounts for 5-10% of the total isoflavones in soy food products. Cell proliferation studies on the dietary phytoestrogen, glycitein against human breast carcinoma SKBR-3 cells showed that glycitein exhibits biphasic regulation on SKBR-3 cells. At concentrations of less than 10 mg/mL, cells respond to glycitein by increasing cell growth and de novo DNA synthesis whereas the addition of glycitein at concentrations greater than 30 mg/mL significantly inhibited cell growth and DNA synthesis in a dose-dependent manner. Cells treated with 60 mg/mL of glycitein did not regain normal growth after treatment was stopped. Glycitein was found to be cytostatic at low concentrations and cytotoxic at higher concentrations. Treatment with 100 mg/mL of glycitein severely altered the cell morphology. Collective results showed that glycitein damaged the cell membranes by increasing membrane permeability and suggested possible mechanisms of the action of dietary phytoestrogens on human breast carcinoma SKBR-3 cells.